Gimme swelter

Scientists discover which circle of hell you’re living in

Photo By Larry Dalton

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In July, Sacramento’s most torrid month, weather-service thermometer readings usually peak around 93 degrees Fahrenheit, though the city can get much hotter. The record books show occasional readings of 105, even 114 during summer heat waves.

Just north of downtown, though, where the railroad tracks gleam in the bright light and the sun beats down on parking lots surrounding large warehouses, it’s like a heat wave many summer afternoons. You can feel the warmth rising from the pavement. You can see it in rippling layers of mirage. It’s that way in other parts of town, too—sweltering, oppressive. Because of what’s known as the urban-heat-island effect, some parts of town are routinely warmer than other nearby neighborhoods.

In the decades since foliage and open fields started making way for asphalt and steel, temperatures have been increasing in human habitats the world over. City centers are often 5 to 10 degrees Fahrenheit warmer than the rural environs surrounding them. Experts say the increased temperatures are because of two major factors: surface albedo, the measurement of a surface’s ability to reflect the sun’s rays rather than absorb them; and trees.

Building and road surfaces respond to solar radiation differently than do trees and greener areas. Low-albedo areas, such as dark asphalt and tar or buildings painted dark colors, tend to absorb more heat than do light-colored rooftops, pale concrete sidewalks or grass-covered parks. The low-albedo areas retain the sun’s heat and then re-radiate it back into the environment even after the sun has started setting.

The other factor in urban heat islands is vegetation—specifically trees, which often are absent from industrial areas.

Trees reflect solar radiation, provide shade and filter pollutants from the air. Evapotranspiration also is important in reducing ambient air temperature.

“Trees moderate urban climate,” said forest meteorologist Jim Simpson, director of environmental analysis at the Center for Urban Forest Research in Davis. He began studying the relationship between vegetation and urban environments about 10 years ago, when he realized that some of the same processes that went on in forests were happening in cities where trees were present and that those processes had important consequences.

“The forest is important in natural areas,” he said, “but it’s also important—maybe more important—where we live every day.”

On a clear June day a few years ago, Jeff Luvall, a senior research scientist at NASA’s Marshall Space Flight Center, flew over Sacramento in a jet equipped with a heat sensor. The purpose of the flight, part of an urban-heat-island-mitigation project, was to produce an image showing the heat coming out of the city and to find out how reflective different parts of town were. The flight produced a picture of Sacramento locations colored dark blue, dark green, light green, orange, red or white depending on their temperatures.

“A park was nice and cool, and you could see the river, and you could see the roofs of the buildings that were dark-colored and very hot,” Luvall said of the flight. “The hottest temperatures were building roofs. They were as high as 170 degrees Fahrenheit. The river was [61 to 63 degrees Fahrenheit]. Trees were very cool, too.”

In general, as Luvall and other researchers expected, downtown Sacramento was warmer than many residential and mixed-use areas surrounding it.

In 1998, a NASA crew recorded this thermal image of Sacramento. White spots represent the rooftops of buildings, the hottest spots in the city at up to 170 degrees Fahrenheit. Red and orange spots, such as the Union Pacific Railyard, are very hot, as well. Green represents cooler areas, such as Capitol Park and tree-covered neighborhoods, and blue represents the coolest areas, including the rivers, which were about 60 degrees Fahrenheit. <br><a href="/issues/sacto/2003-06-05/map.pdf">Click here for a larger map</a>

A bookshelf-sized reproduction of the thermal image hangs on a wall within the offices of the Sacramento Tree Foundation, the organization that coordinated the city’s recent two-year Cool Community program and that worked with NASA and the Environmental Protection Agency to conduct Luvall’s flight. As executive director of the foundation and as the city councilman for District 1, Ray Tretheway has been watching Sacramento’s thermometers for years. Among other things, he’s concerned with air-quality problems and energy consumption, both of which are tied to temperature.

He said downtown is the area hardest hit by stifling summer sun.

“You start downtown on an 85-degree day, and you station somebody in downtown on an axis going straight out,” he said. “Let’s say the first person’s standing in Midtown, the next person’s in the Fabulous 40s, the next person another mile, two miles out. You see that gradient of temperature going lower as you go out. Universally, it’s hotter downtown. That’s your urban-heat-island effect. It also stays hotter downtown later because the heat is absorbed.”

Before the flyover, downtown Sacramento’s relative warmth already was apparent to some Sacramentans, such as city arborist Martin Fitch. “The core downtown is definitely a warmer place. Any place you’ve got high development is a warmer place. Buildings are being heated, and the buildings are radiating heat. And there are mass amounts of pavement and concrete,” he said. “The way I’ve noticed it is when you’ve got frost-sensitive trees. They are more successful in the warmer areas I described. They tend to suffer from frost more in the outlying areas.”

In July and August, some parts of town might be best to avoid altogether. Those show up in bright red, the color signifying “very hot,” on the image created by the NASA flyover.

“The Union Pacific Railyard downtown is by far one of the hottest,” said Tretheway, pointing at the map. “There’s no grass. There’s no shrubs. There’s nothing there, so that’s one of the hottest spots in town. Another hot area would be Arden Fair. There’s no shade trees around that huge parking lot.”

Senior city planner Jim McDonald also was involved with the Cool Community program. He has worked with the city to develop its Smart Growth principles, which, among other things, encourage the use of “cool community” design features such as cool roofing and parking surfaces and ample shade trees. He also is updating the city parking-lot ordinance to create more shade. In 1983, the city adopted an ordinance requiring 50-percent shading in all new off-street parking lots within 15 years of their creation. Studies conducted by the Center for Urban Forest Research in 2001 showed the city wasn’t meeting the 50-percent target, though, so city officials are amending the ordinance further.

McDonald said the hottest parts of town often are in commercial corridors where there are large parking lots built prior to 1983.

Those include Freeport Boulevard, Franklin Boulevard and the rail yards just north of the central business district, he said, though new development would have to be done according to the updated ordinance.

Suburban streets also can be on the warm side if front-yard shade trees haven’t matured yet, McDonald said.

When choosing an outdoor spot for a picnic, tree-lined parks are the obvious best option. Most people know to avoid areas with no shade. But Tretheway said people should give some thought to their clothing and even their transportation during summer. “Being in a white car vs. a dark car helps,” he said.

To demonstrate, Tretheway brought out a hand-held infrared thermometer called a Heat Spy, which he used to gauge conditions in the Sacramento Tree Foundation’s parking lot.

Ray Tretheway, a city councilman and executive director of the Sacramento Tree Foundation, measures surface temperatures with a hand-held infrared thermometer called a Heat Spy.

Photo By Larry Dalton

“It reads surface temperature, not ambient air temperature,” he said. “So, here’s a light car. It’s 87 degrees. Here’s a dark car: 134. That’s the difference. Isn’t that amazing? That was right next to each other. … The inside temperatures are incredibly different in these two cars right now.”

A similar temperature difference existed between the dark parking-lot surface and the pale sidewalk at the edge of it.

For the most part, the cooler pockets of cities make use of reflective surfaces and tons of trees. And Sacramento’s downtown is better off than many cities’ simply by virtue of its many oaks, maples and sycamores. Fitch said the total tree population within the city is close to 2.7 million trees.

Because of the shade those trees provide, older neighborhoods, such as Land Park, Curtis Park and East Sacramento, are cooler, he said.

In a 1999 study, researchers at Ernest Orlando Lawrence Berkeley National Laboratory determined that 13 percent of Sacramento’s overall land area had tree cover—mostly in the downtown area and residential neighborhoods. Up to 23 percent of downtown Sacramento had tree cover. Carmichael (20.5 percent), Tahoe Park (23.5 percent), Del Paso Heights (20 percent) and other residential areas also had a lot of shade from tree canopies.

The Jack Davis Park, Pocket Road and Hagginwood Park areas were lower, at 14.5 percent, 12 percent and 11 percent respectively. Richards Boulevard and Cal Expo were at 8 percent. Only 3 percent of the areas surrounding Florin Road and the Port of Sacramento had tree cover, and the town of Elk Grove was lowest, at 1.5 percent.

“Older Sacramento had tree cover that was very, very large,” said Haider Taha, a staff scientist in the laboratory’s heat-island group. “There are streets in Sacramento you can’t see from above the trees. If you fly over them in airplanes, the streets are invisible because the trees are so huge. That part of Sacramento is quite atypical. We haven’t seen something like this in other places consistently. That part of Sacramento, the tree cover was greater than 70 percent, I think. If you look at all of Sacramento, though, with all suburbs and newer areas, on average, it’s similar to other areas.”

Taha’s group also has noticed a .4-percent-per-decade increase in temperature within Sacramento in recent years. But the city’s characteristic vegetation, colored dark green on the thermal image taken during Luvall’s flight, helps to mitigate that.

“In some parts of the downtown, the older parts,” Luvall said, “you can see the effect of the nice big trees that were planted, and the way the city was laid out. You look right across the river at West Sacramento, and that was one of the hottest places in the city because it just didn’t have the trees.”

Because of the relationship between tree cover and temperature, heat-island experts recommend increasing the number of trees within the city dramatically in the coming years. The Sacramento Tree Foundation’s 2000 State of the Trees Report states that “planting 3 to 5 million more trees in the Sacramento region will contribute to a 2 degree to 3 degree Fahrenheit reduction in afternoon air temperatures and a 7-percent reduction in peak ozone concentrations.” Foundation members hope to plant a million trees per decade for the next 50 years.

The Berkeley lab’s case study called Sacramento a “fairly green city” but said the city had potential for far more urban vegetation: “As climate and air-quality simulations have indicated, 24 million additional trees can have a significant impact on cooling Sacramento and improving ozone air quality.”

With an urban forest of that magnitude, our city corridors might just be bearable year-round—in any neighborhood. Perhaps, even in summer, people would want to get out and enjoy the Mediterranean climate that makes Sacramento such a good place to study heat.